The present invention relates to devices for detecting, monitoring and/or measuring low-level concentrations of certain materials within a complex matrix stream or aggregation of flowable materials. In one of several, more particular aspects, the present invention relates to such devices which employ permeation membrane or diffusion membrane tubing in some fashion to collect and isolate these certain materials from within the stream or aggregation.
Tubular membranes have been suggested for use in this capacity in a number of analytical devices, and are particularly of interest for use in separating out those components of a complex stream which might adversely affect a gas or liquid chromatograph or other analytical device if one attempted to analyze the complex stream directly. Exemplary of these devices are those described in U.S. Pat. Nos. 4,715,217 to Coyne et al. and 4,944,180 to Tou et al., both of which are commonly-assigned and owned with the present invention.
The Coyne et al. patent relates a membrane-assisted analytical chemistry method for determining the concentration of an organic compound in an aqueous matrix, in which a tubular membrane is encased in a protective perforated stainless steel tube and placed in a magnetically stirred sample container. A receiving fluid passes through the selectively permeable membrane tubing and collects the organic compound permeating through the membrane, and transports the organic compound to an analytical means for determining the concentration of the organic compound in the stirred sample.
U.S. Pat. No. 4,944,180 to Tou et al. describes a system for measuring the permeation rate or organic molecules across a polymer film, in which a flow-through semipermeable tubular membrane probe provides an interface to a mass spectrometer. The probe consists of a semipermeable tubular membrane which extends coaxially through an elongated cylindrical bore of the tee. The side arm of the tee is connected directly to the mass spectrometer. Organic molecules permeating through the film being tested are carried through the membrane, and these organic molecules on permeating from within the membrane are communicated to the mass spectrometer.
From a process control and monitoring perspective, each of these analytical devices is sub-optimal in that samples must be drawn from a given process stream and then analyzed. The collection of the materials of interest through a membrane is necessarily time-consuming of itself, but the process of drawing samples creates additional delay in assessing changes in the process stream and altering the composition or course of the process stream responsive to these changes.
Still other shortcomings of these devices in a process control context include the possible safety and exposure hazards associated with the increased handling and repeated sample-drawing required in the use of these devices, along with the additional expense in materials and time for sampling and handling samples from a process stream. Additionally, in many instances, it may be difficult or impossible to draw an appropriate sample even if the drawing could be safely done. The compound of interest could be unstable, and could be lost between the process vessel and the device. Or, the process may be under vacuum or at high or varying pressures.
Tubular membranes have been utilized in on-line systems for the direct, real-time monitoring of certain low-level materials in a matrix stream of materials, but in limited circumstances. For example, in U.S. Pat. No. 4,832,034 to Pizziconi et al., a length of capillary membrane is inserted directly into the blood stream of a patient for monitoring biological reactions and trends. A similar device is described in Brodbelt et al., "In Vivo Mass Spectrometric Determination of Organic Compounds in Blood with a Membrane Probe", Anal. Chem., 1987, Vol. 59, pages 454-457, wherein a membrane probe is supported internally over a portion of its length by the insertion of a nylon monofilament and is used in vivo.
Other membrane-covered probes have been used apparently for monitoring and controlling the reactions in bioreactors. In Cox, "Membrane Inlets for On-Line Liquid Phase Mass Spectrometric Measurements in Bioreactors", Mass Spectrometry in Biotechnological Process Analysis and Control, Plenum Press, 1987 at pages 63-65 is described a perforated stainless steel tube which is capped at one end and which is inserted into the core of silicone rubber tubing. Other devices are described which appear to be of a fundamentally similar nature.
The tubular membrane devices briefly described in the preceding two paragraphs possess a common shortcoming, however, in that the membranes in each are exposed to abrasion or cutting by particulates or other suspended debris (e.g., flakes of rust from the inside of a process pipe) in a matrix stream, or to damage or destruction in pinch points within a pipe, reactor or other process vessel. Further, the influence of the flowing sample stream along and around the membranes of these devices may cause an elongation and distortion of the membranes, and this elongation and distortion of the membranes in turn can be expected to alter the response of the membrane-detector system.
In sum, there is a significant need for an effective tubular membrane-based device for the on-line detecting, monitoring and/or measuring of low-level concentrations of certain materials within a complex industrial process stream, in which the membrane is protected from damage by the debris often found in such environments and is not distorted by a high flow rate in the stream.